Calcifying organisms are marine life forms that produce calcium carbonate (CaCO₃) as part of their biological processes. They use calcium from seawater to build shells and skeletons, contributing to coral reefs and influencing oceanic carbon cycling. Their survival is affected by changes in ocean chemistry.
Diverse Forms of Calcifiers
Calcifying organisms are diverse, contributing to marine environments in distinct ways. Reef-building corals are tiny polyps that secrete hard, stony skeletons, forming complex reef structures. Deep-sea corals also build skeletons, creating habitats in colder, darker waters.
Mollusks, including clams, snails, and oysters, construct protective shells of calcium carbonate. Crustaceans like crabs and lobsters incorporate calcium carbonate into their exoskeletons for support. Echinoderms, such as sea urchins and starfish, possess an endoskeleton made of calcified plates.
Microscopic plankton also engage in calcification. Foraminifera are single-celled organisms that create intricate calcium carbonate shells. Coccolithophores are tiny algae covered in delicate calcium carbonate plates called coccoliths.
How Calcification Works
Marine organisms construct calcium carbonate structures by drawing dissolved calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻) from seawater. This process, known as biomineralization, involves regulating ion uptake and deposition within their tissues. The primary forms of calcium carbonate synthesized are aragonite and calcite.
Corals, for example, primarily build skeletons using aragonite, a less stable form of calcium carbonate that is more soluble in seawater. Many mollusks and some algae also utilize aragonite for their shells. Conversely, organisms like foraminifera and coccolithophores often form structures from calcite, a more stable and less soluble polymorph of calcium carbonate.
Organisms employ specialized cellular mechanisms to facilitate this process, often creating an isolated microenvironment where ion concentration can be precisely controlled. This internal regulation allows them to precipitate calcium carbonate even when external seawater conditions are less favorable. This activity enables growth and protection.
Their Vital Role in Marine Ecosystems
Calcifying organisms play an important role in marine ecosystems. Reef-building corals serve as foundation species, constructing complex habitats that provide shelter, foraging grounds, and nurseries for an estimated 25% of all marine species. These reef structures support significant biodiversity, from small invertebrates to large fish populations.
Calcifiers also contribute to the oceanic carbon cycle. As they form shells and skeletons, they sequester dissolved inorganic carbon from the water, converting it into solid calcium carbonate. When these organisms die, their calcified remains sink to the seafloor, contributing to marine sediments and long-term carbon storage. This process helps regulate atmospheric carbon dioxide levels over geological timescales.
Many calcifiers also form a foundational part of marine food webs. Microscopic calcifiers like coccolithophores are primary producers, converting sunlight into energy and forming the base of many ocean food chains. Their abundance directly influences food availability for zooplankton, which in turn support larger marine animals.
The Growing Threat of Ocean Change
Calcifying organisms face increasing pressure from changes in ocean chemistry, primarily driven by human activities. Increased atmospheric carbon dioxide (CO₂) from burning fossil fuels dissolves into the ocean, leading to ocean acidification. When CO₂ enters seawater, it reacts with water to form carbonic acid (H₂CO₃), which then dissociates into bicarbonate ions (HCO₃⁻) and hydrogen ions (H⁺).
The release of these additional hydrogen ions lowers the ocean’s pH, making it more acidic. A key consequence for calcifiers is the reduction in carbonate ion (CO₃²⁻) concentration in seawater. Carbonate ions are consumed by excess hydrogen ions, forming bicarbonate, making them less available for organisms to build their calcium carbonate shells and skeletons. This scarcity forces calcifiers to expend more energy, leading to slower growth rates and thinner, weaker shells.
Rising ocean temperatures also pose a significant threat, particularly to reef-building corals. Prolonged exposure to elevated temperatures can cause corals to expel symbiotic algae, an event known as coral bleaching. Without these algae, corals lose their primary food source and often die, leading to widespread reef degradation. The combined effects of acidification and warming create a complex challenge for these marine inhabitants.
Wider Impacts of Their Decline
The decline of calcifying organisms has significant impacts throughout marine ecosystems and extends to human communities. Degradation of coral reefs, for example, results in habitat loss for countless fish, invertebrate, and plant species that depend on these structures for shelter and food. This loss of biodiversity can lead to population declines across various trophic levels, disrupting the ecological balance of tropical oceans.
Fisheries, which rely heavily on healthy reef ecosystems, would experience significant impacts as fish stocks diminish. Many commercially important species use coral reefs as breeding or feeding areas. Coastal protection would also be compromised, as reefs act as natural barriers, dissipating wave energy and reducing the impact of storms on shorelines. Their erosion would leave coastal communities more vulnerable to storm surges and sea-level rise.
A widespread decline in calcifiers could also disrupt the global carbon cycle. With fewer organisms sequestering carbon into their shells, the ocean’s capacity to absorb atmospheric CO₂ might be reduced, potentially exacerbating climate change. The economic and social implications for human communities, particularly those dependent on fisheries and coastal tourism, would be significant, affecting livelihoods and cultural practices worldwide.